Electrochromic metal oxides

ABSTRACT

Tin oxide doped with an electrochemically effective amount of a metal, e.g., antimony or niobium, which provides a color change when the doped tin oxide is exposed to an electrochemical potential in the presence of mobile ions. Particles of electrochromic doped tin oxide, e.g., coated on a white or pastel pigment substrate, have a contrast ratio greater than 1.2, where contrast ratio is a measure of electrochromic functionality of a material and is the ratio of reflectance of the material in an oxidized state to the color of the material in a reduced state and where color is a photodiode measurement of the value of light reflected off the oxidized or reduced material from a constant source of light shining on the material. Such doped tin oxide-containing particles are useful as electrochromic material in display devices, and may be employed in display devices that advantageously exhibit a linear or nearly linear relationship between stored electrical charge and voltage, and which also exhibit linear or nearly linear optical characteristics as a function of voltage. Electrochromically active materials made from metal oxides and doped with substances other than antimony and niobium have also been found with high contrast ratios. These additional doped materials may be substituted for electrochromically active antimony-doped tin oxide in electrochromic displays.

This application is a divisional of prior U.S. application Ser. No.08/721,506 filed Sep. 26, 1996 now U.S. Pat. No. 5,876,633 which claimspriority to Provisional application Ser. No. 60/009,226 filed Dec. 26,1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Disclosed herein are novel electrochromic materials comprising dopedmetal oxide-containing particles having a high contrast and methods ofmaking and using such materials, e.g., in novel electrochromic devices,wherein at least some of such devices are characterized by asubstantially linear relationship between an amount of electrical chargestored and a voltage between terminals of the electrochromic device.

2. Description of the Prior Art

Doped tin oxides are known among metal oxides for their relativetransparency and high electrical conductivity. These properties areadvantageously employed in a variety of electro-optical applications,e.g., providing transparent conductive coatings on particles orsurfaces. One such application is the fabrication of transparentelectrodes on electrochromic display devices which typically have anelectrolyte material in contact with an electrochromic material so thatan electrochromic effect is generated when an electric potential isapplied across the interface of the two materials. When electrodes areprovided on both sides of the materials, e.g., in sandwich-likestructure, the electrode on at least one side of the display laminateneeds to be relatively transparent to permit observation of theelectrochromic effect. In such devices typical electrochromic materialsinclude tungsten oxide, Prussian Blue, polyaniline and viologens.Transparent electrodes have been fabricated by vapor deposition of dopedtin oxide coatings on glass or plastic substrates.

Although doped tin oxides have been employed as transparent conductorsin electrochromic devices, it appears that the possibility that dopedtin oxide might be useful as a practical electrochromic material has notbeen discovered. For instance, Orel et al. reported in the Journal ofthe Electrochemical Society, Vol. 141, page L127 (1994) that a film ofATO exhibited a change in light reflectance between a reduced andoxidized state of less than 5%, which corresponds to a contrast ratio(as defined hereinbelow) of less than 1.05. Because such a change incolor is not readily discernable to the typical human eye, it has notbeen recognized or discovered that doped tin oxides have usefulelectrochromic properties.

A variety of dopants are used to make conductive metal oxides, some ofwhich, e.g., fluorine-doped tin oxide and tin-doped indium oxide (ITO),have not heretofore been known to produce materials exhibiting anyuseful electrochromic effect. Similarly, ATO, when provided in filmform, also does not exhibit any useful electrochromic effect.

Furthermore, in prior art electrochromic devices, Faradaic processes,e.g., metal deposition, takes place, causing the prior art devices tohave a non-linear relationship between charge and voltage. TheseFaradaic processes cause charge to increase rapidly after a certainthreshold voltage value is reached, as discussed by B. Conway in TheElectrochemical Society Proceedings, Vol. 95-29, "ElectrochemicalCapacitors," p. 15, a phenomenon that increases the difficulty incontrolling electrochromic displays, because neither charge nor color isa linear function of voltage in such a display. It would be desirable toprovide an electrochromic display free from electrical effects caused byFaradaic processes, so that an amount of charge injected into thedisplay would be directly proportional to the applied voltage over auseful voltage range. To the extent that the color change of the displaywould also be directly and linearly related to the amount of injectedcharge, the color change, as well, would be linearly related to voltage,which would permit the use of simplified display control circuitry.

SUMMARY OF THE INVENTION

When select doped metal oxides, e.g., ATO and niobium-doped tin oxide,are provided in particle form in an electrochromic generatingenvironment, a surprising electrochromic effect is achieved. Thus, thisinvention is directed, in part, to the surprising discovery that certainof the conductive doped metal oxides can be useful high contrastelectrochromic materials and to electrochromic devices employing suchelectrochromic doped oxides.

This invention provides novel electrochromic materials comprising dopedmetal oxide having a high contrast ratio between different oxidationstates. These high contrast electrochromic metal oxide materials aredoped with an electrochromically-effective amount of an ion thatprovides a color change when exposed to ion transfer in an electricfield. Preferred dopants for tin oxide are antimony, niobium, andfluorine. Mixed tin(II)-tin(IV) oxide has also been found useful, as hastin-doped indium oxide.

The invention also provides methods of making such electrochromic metaloxide materials, for instance in the case of antimony-doped tin oxidematerials by employing higher levels of antimony than commonly used inconductive tin oxide applications.

This invention also provides methods of advantageously using suchelectrochromic metal oxide materials, e.g., in display devices. Moreparticularly, this invention also provides a method of producing anelectrochromic effect by applying an electrochemical potential to dopedmetal oxide in contact with mobile ions.

This invention also provides particulate antimony-doped tin oxide thatis oxidized or reduced to provide a powder resistivity which is at leasttwo times the powder resistivity of a base antimony tin oxide compound.Such resistive antimony-doped tin oxide is also uniquely colored ascompared to analogous tin oxides that are highly conductive andtransparent.

This invention also provides electrochromic devices comprising such highcontrast, electrochromic metal oxide materials. Such devices aretypically laminate structures comprising a layer of electrochromicmaterial in contact with an ion-supplying electrolyte layer.

In another significant aspect of the invention, it has been surprisinglydiscovered that when the conductive metal oxides of the presentinvention are provided in high surface area particulate form in anelectrochromic device, this device shows a nearly linear relationshipbetween applied voltage and both charge and electro-optic effect withina range of voltages useful for electrochromically switching between asubstantial contrast range of the device. As-such, a device made inaccordance with the invention could employ greatly simplified controlcircuitry, compared to that required by present electrochromic displays,to exploit available ranges of electrochromic effects.

Therefore, according to another aspect of this invention, there isprovided an electrochromic metal oxide device that providessubstantially linear relationships between applied charge, voltage, andoptical effect.

DESCRIPTION OF THE FIGURES

FIG. 1 is a graph showing the relationship of injected charge to voltageapplied to an electrochromic display containing an ink prepared fromATO;

FIG. 2 is another graph showing the relationship of injected charge tovoltage applied to an electrochromic display prepared from MitsuiPasstran 5210;

FIG. 3 is a graph showing the relationship of applied voltage toreflectance as measured by a photodiode for the display prepared fromMitsui Passtran 5210;

FIG. 4 is a graph showing the relationship of injected charge to voltageapplied to an electrochromic display comprising two W-1 squares in a 10%LiCl solution;

FIG. 5 is a graph showing the relationship of injected charge to voltageapplied to an electrochromic display comprising two W-1 squares in 0.5%AgNO₃ /10% KCl solution;

FIG. 6 is a graph showing the relationship of injected charge to voltageapplied to an electrochromic display comprising two PD007 squares in 10%KCl solution;

FIG. 7 is a graph showing the relationship of applied voltage toreflectance as measured by a photodiode for an electrochromic displaycomprising a Prussian blue ink;

FIG. 8 is a graph showing the relationship of applied voltage toreflectance as measured by a photodiode for an electrochromic displaycomprising a duPont 55 ATO-based pigment;

FIG. 9 is a graph showing the relationship of applied voltage toreflectance as measured by a photodiode for an electrochromic displaycomprising heptyl viologen;

FIG. 10 is a graph showing the effect of antimony dopant level oncontrast ratio;

FIG. 11 is a graph showing the effect of firing temperature of ATOpowders on display contrast;

FIG. 12 is a graph showing the relationship of crystallite size growthand firing temperature;

FIG. 13 is a graph showing the effect of ATO loading on contrast ratio;and

FIGS. 14-19 are partial side views of laminates useful as electrochromicdisplays utilizing the various electrochromically active materialsdescribed herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Percentages expressed herein as "%" are mole percent unless indicatedotherwise, e.g. weight percent is expressed as "wt".

As used herein "powder resistivity" means an electrical resistivitymeasured with a two probe apparatus on powder compressed at highpressure in a confined by defined space. The two probes of the apparatuscomprise metal, e.g. stainless steel, rods about 6.5 mm in diameter andextending about 15 mm from the center of a plate, e.g. a disc that is 9mm thick and 5 cm in diameter; the disks are electrically connected toan ohm-meter. The overall length of the extended rods is slightly longerthan the length of a hole in a non-conducting, e.g., acrylic polymer,cylinder reinforced by a surrounding metal, e.g. aluminum, sleeve andhaving a central bore that is slightly larger than the diameter of therods. To measure powder resistivity, the cylinder is mounted on one discwith the rod inserted into the bore; the open bore is partially filledwith particulate material which is compressed by pressing the second rodinto the bore. Resistance is measured by an ohm-meter when the pressureon the powder is 845 kilograms per square centimeter (which isequivalent to 12,000 psi). At that pressure the height of compressedparticles in the bore (H_(p)) is determined by measuring the spacebetween the discs in centimeters with a micrometer. The cross sectionalarea of the rod (A_(r)) is 0.3318 square centimeters. Powder resistivity(ρ) is determined by multiplying the measured resistance by the ratio ofcross sectional area (A_(r)) to height of compressed particles in thebore (H_(p)).

As used herein the terms "oxidized" and "reduced" mean changing thenumber of electrons associated with a valence state of a material bychemical or electrochemical means. An oxidized metal oxide is a materialthat has fewer electrons than the same metal oxide material in itsnatural state. Conversely, a reduced metal oxide is a material that hasmore electrons than the same metal oxide material in its natural state.A common chemical reducing agent is sodium borohydride which is capableof putting electrons accompanied by ions, e.g. protons or other cationssuch as sodium, lithium, etc., into a tin oxide lattice. In a reducedstate, e.g. when saturated with electrons, doped tin oxide has a darkercolor and lower conductivity. Interestingly, the conductivity of bothreduced and oxidized doped tin oxide is significantly lower than theconductivity of doped tin oxide in the natural state. The conductivityof oxidized doped tin oxide is lower because the number of electroncarriers is substantially lower. The conductivity of reduced doped tinoxide is lower because the material is so saturated with electrons thatelectron mobility is impaired. To achieve an electrochromic effect it isbelieved that it is necessary to cause an electrochromic-generatingoxidation or reduction at the surface of the electrochromic material,e.g. typically by applying an electrical potential of 1 volt or less tocause cations and electrons to migrate into or out of the surface layerof the electrochromic material.

As used herein the term "contrast ratio" (CR) describes the differencein color of a material in oxidized and reduced states. Moreparticularly, contrast ratio means the ratio of reflectance of amaterial in an electrochemically oxidized state to the reflectance ofthe material in an electrochemically reduced state, where reflectance isa photo diode measurement of the value of light reflected off theoxidized or reduced material from a constant source of light shining onthe material. A material having a CR=1 would have no electrochromiceffect, that is the light reflected from the material in a reduced statewould be immeasurably different from the light reflected from thematerial in an oxidized state. The electrochromic metal oxide materialsused in the electrochromic devices of this invention have a CR of atleast 1.2 or higher, say at least 1.4 or 1.6. Preferred electrochromicmetal oxide materials of this invention have a CR of at least 1.8 orhigher, say at least 2 or 3. More preferred electrochromic metal oxidematerials of this invention have a CR of at least 4 of higher, say atleast 4.5 or 5.

A material is said to be in an oxidized state when it has fewerelectrons than in the reduced state. For instance, in oxides, antimonyhas two stable oxidation states with two or zero electrons in the outershell. These are denoted as Sb(III) and Sb(V). In the mixed oxidecompound ATO, as naturally formed, the antimony atoms in the metal oxidelattice are at an oxidation state intermediate between III and V withelectrons from the antimony in a tin 5S electron orbital basedconduction band. In ATO the natural blue gray color is believed to bedue to a charge transfer absorption band.

For electrochromic metal oxides, e.g. ATO, I have now shown that thewide ranges of color and conductivity in metal oxide powders arepossible for a given ratio of dopant to base metal by changing theoxidation state of the material, i.e. changing the number of electrons.For instance, in the case of ATO, the number of delocalized electronsassociated with the antimony can be changed by either chemical orelectrochemical oxidation or reduction.

In particular, in oxidized ATO electrons are removed from the materialeffectively transforming the antimony to a species closer to antimony Vwhich has no free electrons and essentially no color; there are nodelocalized electrons remaining to undergo charge transfer. Conversely,in reduced ATO there is an increase in delocalized electrons in themetal oxide, resulting in more charge transfer light absorption andhence more color. In a natural state, e.g. without exposure to anelectrical potential causing oxidation or reduction, natural ATO has aninherently translucent blue gray color where the intensity and hue ofthe color is a function of the antimony in the mixed metal oxide.

In describing dopants it is useful to use mole ratios of metalcomponents in the tin oxide material. Unless otherwise indicated, theamount of dopant in a tin oxide will be expressed as a mole ratio. Forexample, 6% ATO describes tin oxide doped with antimony where the numberof moles of antimony as a percent of the combined moles of antimony andtin is 6%.

Factors affecting contrast ratio of a doped metal oxide include theamount of dopant and the covering power, i.e. ability to absorb light,of optional adjuvant materials such as pigmented particles mixed withdoped metal oxide particles or providing a substrate for a doped metaloxide coating.

Although doped tin oxide is typically considered transparent ortranslucent, especially when applied as a film, particular doped tinoxide can have perceptible color, perhaps due at least in part to theinteraction of light with the particle. Thus, factors affecting colorwithin the realm of routine experimentation include particle size,amount of dopant, crystallite size and dimensional thickness of thedoped tin oxide material. For instance, particles of 10% ATO have whatappears to be a dark gray color. While the dark gray-colored, doped tinoxide can be used in electrochromic devices, the contrast is often notoptimal since reduced tin oxides generally get darker in color, leavinglittle room in the chromatic spectrum for adequate contrast in displayimages. It has been discovered, however, that when doped tin oxide isused in combination with a light-colored pigment substrate, that thelighter color of the pigment imparts a lighter natural color thatprovides significantly greater contrast when the doped tin oxide isreduced and/or oxidized. Thus, in providing material for use inelectrochromic displays, it is often preferred to provide the doped tinoxide with a light-colored adjuvant, e.g. at white or pastel coloredpigment, that will provide a light background color visible through agenerally transparent or at least translucent, doped tin oxide. Thedoped tin oxide and pigment can be provided as a mixture of particles.It is preferable to provide the doped tin oxide as a coating on, or inadmixture with, a light colored adjuvant substrate. Useful adjuvantpigments include titanium dioxide (TiO₂), mica, aluminum borate, silica,barium sulfate and alumina. When doped tin oxide is used in a mixturewith pigment particles, the pigment material is preferably lesselectrochemically active in aqueous electrolytes than is the doped tinoxide. When used with a light colored pigment adjuvant as a substratefor doped tin oxide, the amount of doped tin oxide in the coating is notcritical so long as the particle is sufficiently conductive. Unlessotherwise indicated, the relative amounts of doped tin oxide and pigmentwill be expressed as weight ratio, e.g. a weight ratio of doped tinoxide to pigment substrate in the range of 1:4 to about 4:1. Usefulpigments have particle size of micrometer (micron) scale, e.g. with anominal diameter in the range of about 0.05 to 20 microns and moretypically about 0.2 to 10 microns and more preferably about 1 to 5microns.

When the adjuvant material is particulate TiO₂ --a commonly used pigmentmaterial with exceptionally high covering power--it has been found that6% ATO coated onto TiO₂ in the weight ratio 2:3 ATO/TiO₂ has a CR of1.2. When the antimony in ATO is increased to about 11 mole percent, theCR is 1.6. A number of ATO coated TiO₂ pigments which are commerciallyavailable as conductive metal oxide particles having from 1 to about 13%antimony are useful in the displays of this invention. For instance, alight gray conductive powder comprising 12.25% ATO on 0.2 micron TiO₂particles in the weight ratio of 23:77 is available from MitsubishiMaterials Company Ltd. as W-1 conducting particles. Gray conductivepowders comprising 12.3% ATO on 1 to 5 micron TiO₂ particles in theweight ratio of 33:77 are available from E.I. Dupont de Nemours andCompany under the tradenames "ZELEC®" 1410T and 3410T. Such commerciallyavailable materials having a CR of about 1.6. When the antimony isincreased to 22 mole percent, the CR is surprisingly increased to avalue greater than 2. Thus, one aspect of this invention provides novelATO coated TiO₂ particles having a CR greater than 1.6, e.g. at leastabout 1.8, more preferably greater than 2.

When other pigments with less covering power than TiO₂ are used, e.g.ATO on aluminum borate (at a weight ratio of about 0.5), it has beendiscovered that doped tin oxide materials with an exceptionally high CR,e.t. up to about 4-5 can be produced. More particularly it has beendiscovered that certain commercially available conductive powderscomprising ATO on pigments such as aluminum borate, barium sulfate zincoxide, silica and mica, are surprisingly electrochromic. In particular,a gray conductive powder with a surprisingly high contrast ratio, i.e.greater than 4, is 11.5% ATO on 4 micron aluminum borate particles inthe weight ratio of 54:46 available from Mitsui as Passtran 5210 Type Vconducting particles.

The doped tin oxide materials of this invention can be obtained fromcommercial sources or produced by a well-known methods with appropriateadjustment in materials, e.g. dopant level and the amount and nature ofadjuvant pigment, for optimal electrochromic effect. For instance,ATO-coated TiO₂ according to this invention can be prepared by adding anhydrochloric acid-acidified aqueous solution of antimony chloride (e.g.,the trichloride or the pentachloride) and tin tetrachloride to anaqueous dispersion of TiO₂ particles, with simultaneous addition ofsodium hydroxide to maintain pH at about 2. This process producesnon-conductive metal hydroxide coated TiO₂ particles which are convertedto conductive, doped tin oxide coated particles when heated to liberatewater, e.g., in the range of 300 to 700° C.

This invention provides particulate antimony-doped tin oxide that isoxidized or reduced to provide a tin oxide compound that has a powderresistivity which is at least two times the powder resistivity of a baseantimony tin oxide compound. Particulate antimony-doped tin oxide iscommonly made by thermally treating a mixed antimony and tin compound.For instance, antimony-doped tin oxide can also be prepared byprecipitating mixed hydroxide particles from a solution of mixedantimony and tin followed by thermal treatment, prefetemperat atemperature greater than 350° C., to form particles of base antimony tinoxide compound. The electrical resistivity of base ATO depends on anumber of variables, e.g. particle size and level of antimony dopant,and can typically range from 0.05 to 10 ohm-cm. With such a wide rangeof base powder resistivity it is expected that there can be some overlapwith the range of increased resistivity for oxidized or reduced tinoxide compounds. It has been found that the powder resistivity isincreased more substantially when the doped tin oxide is reduced ratherthan oxidized. For instance, as shown in the following examples, acommercial ATO having a powder resistivity of 0.12 ohm-cm can be reducedto provide a powder resistivity of about 400 ohm-cm or oxidized toprovide a powder resistivity of about 5 ohm-cm. With heat treatment thepowder resistivity can be returned to a value in the range of theoriginal. When such a base antimony-doped tin oxide is oxidized orreduced there is provided a resistive, particulate antimony-doped tinoxide that is characterized as having a powder resistivity which is atleast two times the powder resistivity of said base antimony tin oxidecompound. Preferably such resistive, particulate antimony-doped tinoxide particles will have a nominal dimension in the range of 0.2 to 10micrometers. Such resistive particulate doped tin oxide being oxidizedor reduced will also exhibit desirable electrochromic properties.

When the electrochromic doped metal oxide materials of this inventionare used in electrochromic display devices, such materials are typicallydisposed in a laminate structure, e.g. a layer of electrochromicmaterial in contact with a layer of ion-supplying electrolyte.Alternatively, displays can be fashioned by providing a layer comprisingelectrochromic particles in a electrolyte matrix. Commonly, anelectrical potential is applied across the materials by electrochromicmaterial and electrolyte. Such electric potential causes ions, such asprotons, lithium ions or sodium ions, to migrate into or out of theelectrochromic material, causing the electrochromic effect-generatingreduction or oxidation. Useful electrochromic displays can be preparedusing the electrochromic doped tin oxide materials of this invention byfollowing the display fabrication principles set forth in my earlierU.S. Pat. No. 5,413,739 (which is herein incorporated by reference) orother principles apparent to those skilled in the art.

This invention also provides electrochromic devices useful for displays.Such devices preferably comprise a layer of electrochromic material incontact with an ion-supplying electrolyte layer. In one preferredembodiment the layer of electrochromic material comprises high contrast,electrochromic, doped metal oxide such as doped tin oxide as disclosedherein and dispersed in a transparent or translucent polymer matrix inan amount such that the material is electrically conductive. The polymerof the matrix can comprise any of a variety of common polymers, e.g.preferably a non-brittle polymer such as a tough elastomeric or rubberypolymer such as nitrile rubber, butyl rubber or butyl acrylate, that isamenable to incorporation of dispersed particles of this invention. Thepolymer matrix can be ionically isolative, e.g. a butyl rubber, orionically conducting, e.g. a sulfonated polymer such as sulfonatedpolystyrene or Nafion ionomer. The electrolyte layer is also desirablytransparent or, at least, translucent. While the ion-supplyingelectrolyte material can comprise a salt dissolved in an aqueous ororganic solvent-containing polymeric gel, a preferred electrolytematerial is an ionically conductive, aqueous polymeric gel which cancontain a humectant or hygroscopic filler. Useful hygroscopic materialincludes deliquescent material such as lithium chloride, calciumchloride, glycerine, sodium dihydrogen phosphate or lithiumtrifluoromethyl-sulfonate. A preferred aqueous polymeric gel ispolyacrylamidomethyl-propanesulfonate, known as POLYAMPS.

In such electrochromic devices the electrochromic metal oxide materialserves as an electrode for transporting electrons into or out of theionically conductive electrolyte media. Concurrent with such electrontransfer is the movement of ions across an interface between said layerof electrochromic material and said ion-supplying electrolyte layer. Inpreferred embodiments of this invention the electrodes can be side byside electrodes as disclosed in my earlier U.S. Pat. No. 5,413,739. Suchside by side electrodes are located behind, e.g. hidden by, theelectrochromic layer of the device.

In order for such electrodes to function, they must be connected to anelectrical potential by current feeders, e.g. conductive leads, whichcan comprise any of a variety of conductive materials such as silverink, carbon ink, metal oxide ink or deposition where the metal oxide isa conductive metal oxide such as ATO. Alternatively, the electrodes canbe in a sandwich disposition such that at least one of the electrodesshould be of transparent or translucent material to allow observation ofthe electrochromic effect. Such transparent electrode material ispreferably a conductive metal oxide such as ATO. When used as a currentfeeder, ATO has an optimally high conductivity in the range of 6-10%ATO. When the transparent metal oxide electrode is used in a sandwichtype display, the current feeder is typically an integral film coating.It is believed that ATO in a film form, as compared to the particulatedoped tin oxide materials of this invention, has such a low contrastratio as to be considered non-electrochromic; that is, the contrastratio is less that 1.2.

It has also been discovered that doped conductive metal oxidescontaining dopants other than antimony and niobium that have heretoforebeen believed not to have useful electrochromic properties can beprepared in forms having sufficiently high contrast ratios (i.e.,greater than 1.2, and preferably greater than 1.4) for use inelectrochromic displays. Such materials include tin-doped indium oxide,mixed tin(II)-tin(IV) tin oxide, and fluorine-doped tin oxide inprecipitated or particulate form. Contrast ratios of up to 1.7 can beobtained from these materials. Commercial ITO (tin-doped indium oxide)powder samples were found to have greater contrast ratios when smallercrystalline sizes were used, while a co-precipitate of hydrated tin andindium oxides, when dried, produced a tin-doped indium oxide having acontrast ratios that varied depending upon the pH of the solution fromwhich the hydrated oxides were precipitated, and the drying temperaturesused.

In one significant aspect of the present invention, displays have alsobeen produced by preparing high surface area conductive metal oxidessuch as ATO on silica or alumina substrates, dispersing these in polymerbinders and coating them onto the surface of an electrode such asprinted silver-carbon. When two of these electrodes are immersed in anelectrolytic solution and a DC voltage applied, color changes areobserved, beginning with voltages as low as 0.1 V. When the chargeversus voltage behavior of these electrodes is measured using a PARelectroanalytical system, it has been observed that the voltage on theelectrodes is essentially a linear function of the amount of chargesupplied thereto through a range of voltage within which the display iselectrochromically active, and that the optical effect is an essentiallylinear function of the applied voltage.

Among the structures in which the inventive electrochromically activematerials may be used in this device are a variety of laminates in FIGS.14-19. These laminates comprise a substrate A, e.g., a non-conductivelayer of polyethylene terephthalate (PET) film, coated with a conductivelayer B of one or more electrodes, e.g., metal, metal oxide, conductivepolymer or carbon. Layer C₁ may comprise one of the electrochromicallyactive materials disclosed herein in an electrically conductive,electrochromic composite layer comprising a dispersion of electricallyconductive electrochromic particles dispersed in a polymer matrix. LayerC₂ is an electrically conductive, essentially ionically isolativecomposite layer comprising a dispersion of electrically conductive(non-electrochromic) particles dispersed in a polymer matrix. Layer C₃is a layer of electrochromic material. Layer D is an ionicallyconductive layer, e.g., POLYAMPS gel. Transparent conductor layer E,e.g., an ITO coated film, can serve as an electrode and transparent,insulating layer F, e.g., a PET film, can serve to prevent loss ofelectrolyte from the conductive layer. With reference to FIG. 14, anelectric potential between electrodes B and E will create anelectrochromic effect at the interface of layers C₁ and D. FIG. 2illustrates a display having side-by-side electrodes B, on substrate A.Because the conductivity of the electrochromic composite layer C₁ islower than the conductivity of the ionically conductive layer D, currentwill preferentially flow from one electrode through the electrochromiccomposite layer to the ionically conductive layer to the area above thenext electrode where it will pass in a reverse direction through theelectrochromic layer to the second electrode. Where the electrochromicmaterial changes color with the loss of an anion, the electrochromiceffect will be visible over one electrode. Where the electrochromicmaterial changes color with both the gain and the loss of an anion,electrochromic effects will be visible over both electrodes.

FIGS. 16, 17, 18, and 19 illustrate bipolar electrodes. In FIGS. 16 and17, an elecrtical potential difference across the outer electrodes willgenerate bipolar potential differences at different halves of theintermediate electrodes so as to create opposite electrochromic effectsin the interface of layers C₁ and D over the bipolar charged ends ofeach intermediate electrode. In FIG. 18, the opposite electrochromiceffects are created at the interface between layers C₁ and D under theedges of the segmented electrolyte layer D. In FIG. 19, the oppositeelectrochromic effects are created at edges of segmented sectionscreating the interface of electrochromic layer C₃ and the ionicalllyconductive layer D.

While the following examples illustrate the preparation and use ofvarious embodiments of the electrochromic doped tin oxides andelectrochromic displays of this invention, it should be clear from thevariety of the examples herein that there is no intention of so limitingthe scope of the invention. On the contrary, it is intended that thebreadth of the inventions illustrated by reference to the followingexamples will apply to other embodiments which would be obvious topractitioners in the electrochromic arts.

EXAMPLE 1

This example illustrates one embodiment of an electrochromic deviceaccording to this invention using commercially available ATO coated TiO₂particles. 1.5 grams (g) of light gray conductive powder comprising12.25% ATO on 0.2 micron TiO₂ particles in the weight ratio of 23:77from Mitsubishi Materials Company Ltd and identified as W-1 conductingparticles was dispersed in 5 g of a 10% solution ofstyrene-butadiene-styrene (SBS) rubber in toluene. A copper coatedpolyester film was used as a electrode substrate. The dispersion wascoated as a film onto the copper layer and dried with a heat gun toevaporate the toluene solvent. The coated substrate was immersed in anaqueous electrolyte solution comprising 5% sodium sulfate. With theapplication of 1 to 2 volts, the coating turned a visibly darker graycolor than the original color of the coating. Reversing the polaritycause a rapid reversal to a lighter gray color that was visibly lighterthan the original color of the coating.

EXAMPLE 2

This example illustrates the fabrication of an electrochromic displaydevice. A first display conductor pattern was printed in the shape of a25 millimeter (mm) square centered on a supporting substrate ofpolyester film with a narrow conductor lead running from the 25 mmsquare to the edge of the polyester substrate. A counter electrodeconductor was printed in the shape of a 12 mm wide line bordering thesquare pattern and lead at a distance of about 1 millimeter from theedge thereof. Each conductor pattern was printed with a conventionalsilver ink and coated with a conventional carbon ink. An electrochromicdisplay was fabricated by overcoating the conductor pattern with adispersion of 12% ATO-coated TiO₂ in a solution of fluorinatedelastomer; the 12% ATO-coated TiO₂ was obtained from MitsubishiMaterials Company Ltd. and is characterized as light gray conductingpowder having a particle size of 0.2 micron with ATO and TiO₂ present inthe weight ratio of 23:77. The fluorinated elastomer was dissolved at 22weight percent (wt %) in butoxyethyl acetate. Sufficient ATO coatedpowder was dispersed in the elastomer solution so that the weight ratioof ATO-coated powder to elastomer was 2.5:1. The conductor pattern wascoated with the dispersion except for the lead portions thereof at theedge of the substrate where electrical connections could be made. Thedispersion coating was dried at 130° C. for 10 minutes, recoated withdispersion and redried to provide an electrically conducting,essentially pin hole-free coating of light gray-colored, electrochromic,doped tin oxide particles dispersed in a transparent, ionicallyinsulating elastomer matrix, designated as a "basic ATO-coated displayelement". The electrochromic layer of the basic ATO-coated displayelement was covered with a stack of adhesive polyester gaskets toprovide an electrolyte well over the electrode area; the well was about1 millimeter (mm) in depth and was filled with electrolyte comprising anaqueous solution of 30 wt % lithium chloride and 5 wt % acrylic polymerthickener, i.e. Acrysol ASE-95 from Rohm and Haas Company. Theelectrolyte filled well was sealed with adhesive polyester film tocomplete the construction of electrochromic display device designatedD1.

EXAMPLE 3

This example illustrates the measurement of contrast ratios forelectrochromic doped tin oxides operating in an electrochromiceffect-generating environment. The conductor leads of the electrochromicdisplay device Dl prepared in Example 2 were connected to a functiongenerator which applied a 50 milliHertz, ±1.5 volt square wave potentialto drive the electrochromic device causing the electrochromic ATO-coatedparticles visible through the electrolyte coating to cycle between alight gray and a dark gray color as the ATO was sequentially oxidizedand reduced. The magnitude of the contrast ratio of the color change wasdetermined by fitting the device under a microscope fitted with aMelles-Griot photodiode and wide band width amplifier and measuring theoutput in millivolts corresponding to the light and dark state. Theratio of millivolts measured in the light state divided by millivoltsmeasured in the dark state provided the measure of "contrast ratio" or"CR" as used herein. The electrochromic, 12% ATO-containing material inthe device exhibited a contrast ratio of 1.38.

EXAMPLE 4

This example illustrates the dramatic effect of an increased amount ofantimony in electrochromic properties of ATO. A basic ATO-coated displayelement prepared according to Example 2 was coated with additionalelectrochromic dispersion and fabricated into an electrochromic displayin essentially the same manner of Example 2 except for the use of 33%ATO. The contrast ratio measured in the manner of Example 3 was 1.92.

EXAMPLE 5

This example further illustrates the dramatic effect of an increasedamount of antimony in electrochromic properties of ATO. A set of basicATO-coated display elements prepared according to Example 2 were coatedwith an additional electrochromic dispersion and fabricated into anelectrochromic display in essentially the same manner of Example 2except that the doped tin oxide comprised antimony in the range of 11 to60 percent and that the ATO and TiO₂ were in the weight ratio of 36:64.The contrast ratio measured in the manner of Example 3 is reported inTable 1.

                  TABLE 1                                                         ______________________________________                                                 % Sb CR                                                              ______________________________________                                                 5.7  <1.2                                                                     11   1.6                                                                      23   2.0                                                                      33   2.14                                                                     43   2.19                                                                     47   2.17                                                                     55   1.74                                                                     60   1.94                                                            ______________________________________                                    

EXAMPLE 6

This example illustrates the dramatic increase in electrochromic effectachieved by selection of substrate pigment. An ATO-coated displayelement prepared similar to the procedure of Example 2 was furthercoated with an electrochromic dispersion of 12% ATO-coated onto aluminumborate (obtained from Mitsui as Passtran 5210 conductive powder) in afluorocarbon elastomer solution. In the dried electrochromic top coatthe weight ratio of ATO-containing particles to fluorocarbon elastomerwas 28:15. An electrochromic device prepared as in the manner of Example2 and evaluated in the manner of Example 3 showed that the doped tinoxide on an aluminum borate substrate had a contrast ratio of 5.14.

EXAMPLE 7

This example illustrates the preparation of an electrochromic,niobium-doped tin oxide according to this invention. A barium sulfateslurry was provided by dispersing 50 g of barium sulfate powder in 750ml of water and heating to 75° C.; the slurry was adjusted to pH 12 with25% sodium hydroxide solution. A tin solution (98.5 g of sodium stannatetrihydrate in 250 ml of water at 75° C.) was added to the slurry. Afterstirring for 30 minutes, an acidic niobium solution (0.735 g niobiumtrichloride in 25 ml methanol acidified with 270 cc of 20% sulfuricacid) was added to the tin oxide-containing slurry over a 90 minuteperiod. The pH of the niobium/tin-containing slurry was adjusted to 2.5with 20% sulfuric acid. After 3 hours the solution was cooled and filterwashed 10 times with 250 ml of water, providing particles that weredried in a vacuum oven at 130° C. The dried particles were calcined for2 hours under nitrogen at 450° C. to provide electrochromic 0.72%niobium-doped tin oxide coated barium sulfate substrate particles havinga contrast ratio of 1.54.

EXAMPLE 8

This example illustrates the utility of mixtures of doped tin oxideparticles and pigment particles as an electrochromic material. A mixtureof 0.4 g of 13.4% ATO particles (commercially available from DuPont as"ZELEC®" 3010XC ATO) and 0.15 g of TiO₂ particles was dispersed in 1 gof 22 wt % fluoroelastomer solution in butoxyethyl acetate to provide adispersion suitable for use in an electrochromic display as describedherein. The material exhibited a contrast ratio of 2.4.

EXAMPLE 9

This example illustrates the high resistance of oxidized or reduceddoped tin oxide particles according to this invention. "ZELEC" 3005XCATO obtained from DuPont was determined to have a base ATO powderresistivity of 0.12 ohm-cm. The base ATO was treated with sodiumborohydride, washed and dried to provide reduced ATO (having 0.58%sodium ions) having a powder resistivity of 404 ohm-cm. The base ATO wastreated with ammonium persulfate to provide oxidized ATO having a powderresistivity of 5.5 ohm-cm. When the oxidized ATO is heated, the powderresistivity returns to a value close to 0.1 ohm-cm.

EXAMPLE 10

This example illustrates the linear relationship between charge andvoltage in an electrochromic device made with ATO in accordance withthis invention. FIG. 1 shows charge and capacitance versus appliedvoltage for two 2"×2" test electrodes printed with silver, carbon,Mitsubishi W-1 and Acheson PD008 (an ink prepared from 75% ATO and 25%silica, with 43 mole percent Sb and 57 mole % Sn in a fluorelastomerbinder) and immersed face to face about 2 cm apart in a beakercontaining 10% LiCl solution in water. The leads of the test electrodeswere attached to a power supply, and the test electrodes equilibrated at25 mv for 20 seconds before each data point was run. Values of appliedvoltage ranged from -1.0 volt to +1.0 volt in 100 mv steps. FIG. 1 showsboth a plot of charge vs. voltage and a plot of capacitance vs. voltagefor the test electrodes. The injected charge is a close approximation toa straight line function of voltage within the range of -2V to +2V,which means that the capacitance of the device is nearly constant overan unusually wide range of voltages. Electrochromic switching of thisdisplay requires voltages of only ±1.3 v, which is well within thelinear charge-voltage relationship.

EXAMPLE 11

This is an example of a capacitor-like charge and voltage relationshipin which a linear electro-optic effect is also demonstrated in anelectrochromic device using Passtran 5210 in accordance with thisinvention. FIG. 2 shows behavior similar to that of Example 10 (i.e., asimilar linear relationship between voltage and charge and a nearlyconstant capacitance over a ±2.0 volt range) for a display prepared fromMitsui Passtran 5210, albeit with somewhat lower capacitance than thatof Example 10. FIG. 2 is a plot of data obtained from test electrodesimmersed in beakers, as in Example 2. FIG. 3 is a plot showing thereflectance of a test display prepared from Mitsui Passtron 5210, inaccordance with a method similar to that used to make the device inExample 2 with reflectance measured in accordance with the technique ofExample 3. A linear relationship between applied voltage and reflectanceis evident over a range from approximately -1.0 volt to +1.0 volt,producing a contrast ratio range of greater than 2.0 within the linearrange.

EXAMPLE 12

This is an example of a capacitor-like charge and voltage relationshipin an electrochromic device using Mitsubishi W-1 in accordance with thisinvention. FIG. 4 show behavior similar to that of Example 10 (i.e., asimilar linear relationship between voltage and charge over a range of±2.0 volts) for a display prepared from Mitsubishi W-1, albeit withsomewhat lower capacitance than that of Example 9.

Aside from the results shown in Examples 10-12, it is expected thatcapacitor-like charge and voltage relationships should be obtained froma wide range of materials other than ATO. Requirements for such arelationship to exist are that the material have relatively highelectric conductivity (e.g., powder resistivity of less than 1,000ohm-cm), high surface area (e.g., at least 10 m² /g or higher, the value10 m² /g being obtained from commercially available materials, withvalues up to 160 m² /g obtained from materials made in laboratoryexperiments), a balance of visible absorption versus light scatteringbehavior such that distinctly different colors are observed for"oxidized" versus "reduced" forms of the powder, and anelectrochemically accessible potential range sufficiently wide thatoxidation and reduction may be carried out without encounteringinterfering Faradaic reactions.

EXAMPLE 13

FIGS. 5 and 6, respectively, show a distinctly nonlinear relationshipbetween charge and voltage when a Faradaic component is added to thedevice of Example 10. Specifically, FIG. 5 shows the nonlinearrelationship between voltage and charge when silver ion is present in atest performed with two squares coated with W-1 ink in a 10% KNO₃solution, and FIG. 6 shows the nonlinear relationship when Prussian bluePD007 is used as an ink on two squares in a 10% KCl solution. Silver ionintroduces a Faradaic reaction into the test, thereby producingdifferent values of capacitance at negative potentials. FIG. 6, in whichPD007 Prussian Blue is printed on two test squares shows distinctlynon-linear charge to voltage characteristics, resulting in a peakcapacitance at about -100 mv. These nonlinear relationships stand incontrast to the capacitor-like behavior shown in Examples 10, 11, and12.

EXAMPLE 14

A test display was prepared using 2.6% Prussian Blue on Dupont 1703S(ATO-silica) in accordance with the testing method of Example 2, andreflectance was measured in accordance with Example 3. The resultingreflectance vs. applied voltage relationship is plotted in FIG. 7. Therelationship shown in FIG. 7 is linear over a range from 0 to about 400mv, where most of the display's contrast range of greater than 2.0 ispresented. Below 0.0 volts, the display exhibits a fully saturated bluecolor, and does not change appreciably as voltage is made more negative.While the range of voltage over which a linear relationship with theobserved optical effect is limited, the display does provide a usefuland substantially linear contrast change within the limited range ofvoltage.

EXAMPLE 15

A test display using duPont 55 [75 wt % ATO (43 mole % Sb, 57 mole %Sn)+25 wt % silica shells] was prepared in accordance with the method inExample 2 and tested in accordance with the method of Example 3. A plotof the results showing reflectance vs. applied voltage is provided inFIG. 8. This plot shows a substantially linear characteristic overapproximately a 2.0 contrast ratio for applied positive voltages of 0.0volt to 1.0 volt, with some deviation from linearity at negativevoltages.

EXAMPLE 16

A test display was prepared using W-1 squares in a 5% heptyl viologensolution in 10 wt. % NH₄ NO₃ water solution. (The heptyl viologenproduces a Faradaic reaction, resulting in a nonlinear relationshipbetween charge and voltage.) The test display was prepared in accordancewith the method of Example 2, and tested in accordance with the methodof Example 3. A plot of reflectance vs. applied voltage is provided inFIG. 9. From ±1.0 volt down to -0.75 volt, the display producesrelatively little contrast change. Only below -0.75 volt does asubstantial contrast change occur, and the contrast change does notappear to be linearly related to the applied voltage, which is to beexpected in electrochromic displays in which Faradaic reactions occur.

EXAMPLE 17

This example shows a non-antimony doped electrochromic metal oxide thatwas previously thought not to exhibit useful electrochromic behavior. Inthis example, three commercial ITO (tin-doped indium oxide) powdersamples were made into inks with fluoroelastomer binder solution, asdiscussed previously, and coated onto printed test electrodes. Theelectrolyte used was 25% LiCl-5% ASE-95 (Rohm & Haas polyacrylate). Theapplied voltage was +1.5 volts AC square wave at 50 milliHertz(mHz)--i.e., 10 second pulses of alternating polarity. The contrastratio of the inks was observed to increase with decreasing crystallinesize of the ITO crystallites, as summarized in Table 2. (Crystallitesize was determined via X-ray diffraction).

                  TABLE 2                                                         ______________________________________                                                  Color at  Color at  Contrast                                                                            Crystal-                                  Sample    -1.5v     +1.5v     Ratio lite size                                 ______________________________________                                        Mitsui    Blue      Greenish  1.7   143Å                                  Passtran            Yellow                                                    ITO                                                                           Sumitomo  Blue      Greenish  1.4   295Å                                  ITO                 Yellow                                                    Spheres                                                                       Sumitomo  Olive-    Olive-    1.1   511Å                                  ITO       Yellow    Yellow                                                    Needles                                                                       ______________________________________                                    

EXAMPLE 18

This example illustrates a method of preparing tin-doped indium oxidethat exhibits a substantial electrochromic effect. Tin-doped indiumoxide was produced by co-precipitation of the hydrated oxides from amixed solution of SnCl₂ (10 mole %) and InCl₃ (90 mole %) in aqueousHCl, neutralized to pH 6.5. After filtering and drying, the hydratedoxide was fired at various temperatures. Contrast ratios weresatisfactory, with higher contrast ratios being obtained when theco-precipitate was fired at lower temperatures and for shorter times.The results are summarized in Table 3.

                  TABLE 3                                                         ______________________________________                                                                           Powder                                     Sample                             Resistivity                                (drying   Color at  Color at Contrast                                                                            (ohm-cm at                                 temp./time                                                                              -1.5v     +1.5v    Ratio 12K psi)                                   ______________________________________                                        400° C./3 hr.                                                                    Blue      Yellow-  1.4   3.5                                                            Orange                                                    600° C./3 hr.                                                                    Blue      Yellow-  1.4   2.3                                                            Orange                                                    850° C./4 hr.                                                                    Blue      Yellow   1.2   1.8                                        ______________________________________                                    

EXAMPLE 19

This example illustrates a method of preparing tin-doped indium oxidethat exhibits a substantial electrochromic effect. Samples were againprepared in accordance with the method of Example 15, except that anincreased amount of SnCl₂ was used (25 mole % SnCl₂, 75 mole % InCl₃).Precipitation was at pH 10.5 rather than the pH of Example 15. Resultsare summarized in Table 4.

                  TABLE 4                                                         ______________________________________                                                                           Powder                                     Sample                             Resistivity                                (drying   Color at  Color at Contrast                                                                            (ohm-cm at                                 temp/time -1.5v     +1.5v    Ratio 12K psi)                                   ______________________________________                                        400° C./3 hr                                                                     Blue      Yellow-  1.5   16.7                                                           orange                                                    600° C./3hr                                                                      Blue      Yellow-  1.5   2.7                                                            orange                                                    ______________________________________                                    

EXAMPLE 20

This example illustrates a tin(II)-doped tin(IV) oxide on alumina thatexhibits useful electrochromic properties. A mixed tin(II)-tin(IV) oxidewas prepared by co-precipitating the mixed chlorides (20 mole % SnCl₂,80 mole % SnCl₄) in aqueous HCl into a slurry of alumina (Aldrich<5micron) at pH 2.0 using NaOH. The co-precipitate was filtered, dried andfired at various temperatures. All operations were carried out in anitrogen atmosphere. Table 5 summarizes the results, showingsatisfactory contrast ratios when firing was carried out at 400° C. and600° C. The sample fired at 800° C. was found to be essentiallynonconductive, and as such, not useful for electrochromic displays.

                  TABLE 5                                                         ______________________________________                                        Sample                             Resistivity                                (drying   Color at  Color at Contrast                                                                            (ohm-cm at                                 temp/time -1.5v     +1.5v    Ratio 12K psi)                                   ______________________________________                                        400° C./3 hr                                                                     light     dark gray                                                                              1.4   6.5                                                  gray                                                                600° C./3 hr                                                                     light     dark gray                                                                              1.3   153                                                  gray                                                                800° C./3 hr                                                                     --        --       --    1.9 × 10.sup.6                       ______________________________________                                    

EXAMPLE 21

This example describes the preparation of a fluorine-doped tin oxidethat exhibits useful electrochromic properties. Hydrated tin oxide wasprecipitated from a solution of SnCl₄ in aqueous HCl by adjusting to pH2.0 at 90° C. with an NaOH solution. The precipitates were filtered,washed, and dried at room temperature. A first sample (a) was preparedby adding 0.5 g of SnF₂ to 50 ml deionized water, mixed with 5 g of thehydrated tin oxide and the mixture evaporated to dryness on a rotaryevaporator. The residual solid was then heated to 600° C. for threehours. A second sample (b) was prepared by repeating the steps used toproduce sample (a), but by using 0.5 g of SnF₄ instead of SnF₂. Displayswere prepared as in the previous example. Results are summarized inTable 6.

                  TABLE 6                                                         ______________________________________                                                 Color at Color at   Contrast                                                                            Conductivity                               Sample   -1.5v    +1.5v      Ratio at 12K psi                                 ______________________________________                                        (a)      dark gray                                                                              light gray 1.5   42.8                                       (b)      dark gray                                                                              light gray 1.6   15.3                                       ______________________________________                                    

EXAMPLE 22

This example illustrates the relationship between antimony dopant leveland contrast ratio. To prepare a sample of ATO on alumina that was 75 wt% ATO (43 mole % Sb, 57 mole % Sn and 25% wt alumina), a solution ofSnCl₄.5H₂ O (Aldrich 98%, 40 g) in distilled water (20 ml) was mixedwith SbCl₅ (Aldrich 99%, 25 g) in concentrated HCl (20 ml). Alumina(Aldrich, log) was stirred with deionized water (200 ml) in a beaker andheated to 90° C. The mixed Sb/Sn chloride solution was added from aburette with concurrent addition of 15% NaOH from another burette tokeep pH at 0.8-1.2 for about 45 minutes. The pH was then adjusted to2.0, the heat turned off, and the mixture left to stir and cool down forthree hours. The solids were then filtered off and washed with about 1.5liters of water. The solids were then dried under vacuum. (Normally,this was done at room temperature, but it was also observed as part ofthis experiment that the samples were not affected by slightly elevatedtemperatures, i.e., 60 to 100° C., during drying). The dried powder wasthen fired in an open ceramic crucible at about 600° C. for three hoursin air. Quantities of about 20 g at a time were fired to ensure uniformheating in a preheated oven. At the end of the firing time, the samplewas removed from the hot oven and allowed to cool quickly in air.

Five other 75 wt % ATO, 25 wt % alumina samples were synthesized by thismethod with varying mole percentages of antimony in the ATO by varyingthe ratio of antimony to tin in the initial mixture. The mixtures wereeach made into displays in accordance with the method discussed inExample 2, and the contrast ratio of the resulting displays weremeasured in accordance with the method discussed in Example 3. Theresults are plotted in FIG. 10. It was observed that the contrast ratiois maximized at an antimony dopant percentage of about 43 mole %, and isreduced as the percentage is either increased or decreased therefrom.

EXAMPLE 23

This example illustrates the relationship between the firing temperatureof 75 wt % ATO (43 mole % Sb, 57 mole % Sn), 25 wt % alumina powders onthe contrast ratio of displays made therefrom. Four samples of ATO onalumina powders of the specified composition were prepared in accordancewith the method of Example 22, except that the firing temperature wasvaried as indicated in FIG. 11. (All samples were fired for threehours). Each sample was made into a display in accordance with themethods of Example 2, and the contrast ratio of each display measured inaccordance with the methods of Example 3. The results were plotted inFIG. 11. It was observed that the best results were obtained by firingat 600° C., although a contrast ratio of approximately 2.0 was obtainedat a firing temperature of 300° C. A firing temperature of 1000° C.produced a substantially less satisfactory contrast ratio.

EXAMPLE 24

This example shows the relationship between firing temperature of pureATO and crystallite size. Two different samples of pure ATO wereprepared in accordance with the method of Example 22, except that theATO was precipitated without the addition of alumina. The two samplesdiffered in that a first sample contained 43 mole % Sb, 57 mole % Sn,and the other sample contained 10 mole % Sb, 90 mole % Sn. These sampleswere further divided into portions, and the portions fired at varioustemperatures up to 1000° C. for three hours. One portion of each samplewas not fired; these portions were used as the 20° C. data points. Theresults are plotted in FIG. 12. It was observed that the 43 mole % Sbsample had somewhat smaller crystallite size than the 10 mole % Sbsample when fired between 350 and 800° C., and that firing at 1000° C.greatly increased crystallite size of both samples. These greatlyincreased sizes show that the highest firing temperatures produce lesssatisfactory product for display use.

EXAMPLE 25

This example shows the effect of varying the amount of ATO loading (43mole % Sb, 57 mole % Sn) on alumina. Samples of ATO-alumina having thisrelative molar doping percentage were precipitated on alumina using themethod of Example 22, but with the relative proportion of aluminavaried. The resulting samples were made into displays using the methodof Example 2, and the contrast ratio measured in accordance with themethod of Example 3. The results are provided in FIG. 13. Although apeak in the contrast ratio was observed at 75% ATO loading, a contrastratio of 2.0 or more was observed for all loadings between 10 wt % and90 wt % of ATO.

While specific embodiments have been described herein, it should beapparent to those skilled in the art that various modifications thereofcan be made without departing from the true spirit and scope of theinvention. Accordingly, it is intended that the following claims coverall such modifications within the full inventive concept.

What is claimed is:
 1. A method of producing an electrochromic contrastratio greater than 1.2 by applying an electrochemical potential toparticulate doped tin oxide in contact with mobile ions, therebyelectrochromically activating said doped tin oxide particles, whereinsaid activated doped tin oxide electrochromically exhibits a contrastratio greater than 1.2.
 2. A method of generating an elcctrochromicreflectance having a linear relationship with an applied voltagecomprising coating an electrode with an electrically conductiveparticulate electrochromic doped metal oxide having a surface area of atleast 10 m² /g , and activating said electrochromic doped metal oxide toexhibit a contrast ratio greater than 1.2.
 3. The method of claim 2wherein the electrochromic doped metal oxide has distinctly differentcolors in its oxidized and reduced forms.
 4. A method for producing achange in electrochromic reflectance of an electrochromic device towhich a voltage is applied comprising changing the applied voltagewherein the change in reflectance is a linear function of the appliedvoltage, said device comprising electrochromically activated particulatedoped metal oxide, wherein said activated doped metal oxideelectrochromically exhibits a contrast ratio greater than 1 2.